The present invention relates to a method of treating waste gas containing poisonous gas components such as SO.sub.x and/or NO.sub.x.
FIG. 1 shows a flow sheet schematically showing a typical process for treating waste gas containing SO.sub.x and/or NO.sub.x which is discharged from a heavy oil combustion furnace or the like to make the waste gas harmless. More specifically, waste gas (generally at 130.degree. C. or more) which is generated from a boiler 1 is introduced through a waste gas duct 2 into a cooling tower 3 where it is cooled down by means of cooling water sprayed from a cooling water supply pipe 4 so that the gas temperature is in the range of from the dew point to 100.degree. C., and the cooled waste gas is then introduced into a reactor 7 through a waste gas duct 5. At the intermediate portion of the duct 5, ammonia is added to the waste gas through a flow control valve 6.
The waste gas introduced into the reactor 7 is irradiated with electron beams from an electron beam generator 9. As a result, the SO.sub.x and/or NO.sub.x contained in the waste gas reacts with ammonia to change into ammonium sulfate and/or ammonium nitrate, which is then removed by means of a dust collector 11, and the purified waste gas is then released into the atmosphere from a smokestack 13. The ammonium sulfate and/or ammonium nitrate removed from the waste gas is collected as a by-product from a discharge pipe 12. It should be noted that, in order to control rise in temperature of the waste gas due to the generation of heat by electron beam irradiation and the generation of heat accompanying desulfurization and/or denitrification and thereby maintain the waste gas at an optimal temperature, the waste gas is sprayed with cooling water in the reactor 7 using cooling water spray means 8 at at least one of the three positions, that is, within the irradiation zone and at the upstream and downstream sides thereof. It is most preferable to spray the waste gas with cooling water after the irradiation (an invention relating thereto was filed as Japanese Patent Application No. 308887/1987).
The dust collector 11 may comprise either an electrostatic precipitator (EP) or a bag filter or both. In the case of using a bag filter alone, since the pressure loss of the waste gas rises within a short period of time, it is necessary to use a bag filter having a relatively large capacity in order to ensure a stable operation, which leads to an increase in the cost.
The reference numerals 14, 15 and 16 in FIG. 1 denote an SO.sub.x analyzer, an NO.sub.x analyzer and a waste gas flowmeter, respectively. The amount of ammonia (NH.sub.3) to be added may be calculated from the flow rate of waste gas (QNm.sup.3 /h), the SO.sub.x concentration ([SO.sub.x ] ppm), the NO.sub.x concentration ([NO.sub.x ] ppm), the target desulfurization efficiency (.eta..sub.SOx) and the target denitrification efficiency (.eta..sub.NOx) as follows: ##EQU1##
In order to reduce emission of poisonous components, waste gas treatment equipment has recently been required to meet extremely stringent emission standards, i.e., desulfurization efficiency: 90% or more; denitrification efficiency: 80% or more; and leakage ammonia: 10 ppm or less. It is expected that the emission standards will become increasingly stringent in the future.
However, the above-described prior art method suffers, from the disadvantage that it is difficult to control the leakage of ammonia, and another problem may arise in regard to by-products. This will be explained hereinunder more specifically. FIG. 2 is a typical chart showing variations in the SO.sub.x and NO.sub.x concentrations in coal combustion waste gas. The SO.sub.x concentration has variations of about .+-.100 ppm with respect to an average value of 1500 ppm, while the NO.sub.x concentration has variations of about .+-.20 ppm with respect to an average value of 300 ppm. The amount of ammonia which is to be added to waste gas in a process wherein the target desulfurization efficiency is 90% and the target denitrification efficiency is 80% is calculated from the expression (1) as follows: ##EQU2##
Assuming that the allowable leakage ammonia concentration is the aforementioned level, i.e., 10 ppm, it is necessary to feed waste gas having an ammonia concentration in the range of from 2744 ppm to 3136 ppm while controlling the leakage of ammonia with tolerances of .+-.10 ppm, that is, from 0.3 to 0.4% (i.e., from 10/3136 to 10/2744).
The values of the required control accuracy are considerably lower than those of the presently achievable control accuracy (i.e., from 1 to 2% of the full scale), so that it has heretofore been difficult to control the leakage of ammonia to 10 ppm or less.
When waste gas is irradiated with electron beams, X-rays are partially generated by bremsstrahlung. As is well known, the rate of generation of X-rays by bremsstrahlung is quite small, but, since the range of the generated X-rays is longer than that of electron beams, the X-rays must be shielded by means, for example, of lead or concrete.
FIG. 7 illustrates the structure of a typical irradiation chamber of an apparatus for carrying out the waste gas treating method. In this apparatus, the electron beam irradiation sections 9' of the electron beam accelerator and the reactor 7 are installed in an irradiation chamber 17 made of a shielding material, for example, concrete, as shown in FIG. 7, in order to provide a shield against X-rays resulting from the irradiation with electron beams. A maze (zigzag passage) 19 is defined by the shielding materials, and ducts 5' and 10' for introducing waste gas into the reactor 7 and discharging the gas irradiated with electron beams from the reactor 7 are disposed zigzag fashion along the maze 19, thereby allowing electron beam irradiation to be carried out with a shield provided against X-rays. The maze 19 is generally arranged in such a manner that the passage includes two or three right angle formations, thereby preventing X-rays from leaking out of the irradiation chamber 17.
X-rays that penetrate into the shielding material decay therein but some of them are reflected. The intensity of the reflected X-rays is exceedingly weak, i.e., one/hundredth to one/thousandth of their original intensity. Accordingly, it is possible to provide an effective shield against X-rays by means of a maze having a structure in which an integral passage includes two or three right angle formations.
The waste gas that contains dust particles of a by-product produced by the irradiation with electron beams is introduced into the by-product dust collector 11 outside the irradiation chamber 17 through the maze-like duct 10 having such a structure that the duct includes two or three right angle formations. Inside this maze-like duct, particularly the portions which are formed into right angles, adhesion and deposition of dust are found to occur, and these lead to an increase in pressure loss of the waste gas being treated and unstable operation.
The by-products, which consist mainly of ammonium sulfate and ammonium nitrate, represent a useful nitrogen fertilizer but, when the CO concentration in waste gas is below 10 times the SO.sub.x concentration, sulfamic acid, which is harmful to plants, is generated, and although the content of sulfamic acid is very low, i.e., several percent, the fertilizer formed from the by-products, inhibits the growth of plants [the present applicant has already filed a method of removing sulfamic acid compounds by thermal cracking under Japanese Patent Application No. 61-279791 (1986)].
The present inventors conducted various studies in order to reduce the leakage of ammonia and modify the by-products without lowering either the desulfurization efficiency or the denitrification efficiency. As a result, we have found that it is possible to attain this objective by adding an amount of ammonia which is smaller than that which is needed to achieve target desulfurization and denitrification efficiencies and further adding an alkaline substance exclusive of ammonia to remove the SO.sub.x and NO.sub.x which are left unreacted due to lack of ammonia, thereby modifying the by-product while maintaining the overall desulfurization and denitrification efficiencies at high levels.
It is also possible to substantially inhibit adverse effects of sulfamic acid impurities by adjusting the pH of the collected by-product to 6 or more, preferably 7 or more.